I'm using Linux Mint and disabled the touchpad, and like others before me, I noticed how hard it is to switch it back on if you don't have a mouse handy.
Using the Mint Preferences to disable the touchpad won't let you enable it again by using xinput or synclient as suggested in various places.
tl;dr
gsettings set org.cinnamon.settings-daemon.peripherals.touchpad touchpad-enabled true
Longer version on how I managed to get it back.
I dumped the cinnamon config to a file with:
'dconf dump /org/cinnamon/ > mysettings'
Under the heading 'settings-daemon/peripherals/touchpad' i found 'touchpad-enabled=false'
List the schemas and find something related to the touchpad
gsettings list-schemas | grep touchpad
List the keys in the touchpad-schema
gsettings list-keys org.cinnamon.settings-daemon.peripherals.touchpad
Enable the touchpad
gsettings set org.cinnamon.settings-daemon.peripherals.touchpad touchpad-enabled true
Some useful links: